An optical circulator is a multi-ported passive device designed to receive as an input an optical signal on one port and transmit the optical signal to another port. Conventional optical circulators are employed in systems that require the transmission of an optical signal in a particular direction. For example, U.S. Pat. No. 4,650,289 by Kuwahara describes a conventional optical circulator. FIG. 1 is a diagram of one such conventional optical circulator 10. The conventional optical circulator 10 includes four ports, port A 12, port B 24, port C 32, and port D 34. The conventional optical circulator 10 also includes polarizer prisms 14 and 22, mirrors 16 and 26, Faraday rotators 18 and 28, and optically active elements (e.g., half wave plates) 20 and 30. Polarizer prisms 14 and 22 transmit light in different directions depending on the polarization of the light. Light polarized in a first direction is transmitted undeflected by the or polarizer prisms 14 and 22. Light polarized in a second direction is transmitted at an angle of ninety degrees from the first direction. The mirrors 16 and 26 merely reflect light without a change in polarization. The Faraday rotators 18 and 28 rotate the direction of polarization of incident light by forty-five degrees in a particular direction regardless of the direction in which light traverses the Faraday rotators 18 and 28. For example, the Faraday rotator 18 rotates the polarization of light from the mirror 16 in the same direction as light from the optically active element 20. Optically active elements 20 and 30 rotate the polarization of incident light by forty-five degrees. However, the direction that the polarization is rotated depends upon the direction in which the light traverses the optically active elements 20 and 30 (i.e., optically active elements 20 and 30 are reciprocal devices). For example, optically active element 20 will rotate light from the Faraday rotator 18 by forty-five degrees in a particular direction. The optically active element 20 will rotate light from the polarizer prism 22 having the same polarization by forty-five degrees in the opposite direction. The Faraday rotator is an optically irreversible (i.e., non-reciprocal) element, that is, the rotation angle will double for light after a round trip through the Faraday rotator. Optically active elements 20 and 30 are reciprocal, that is, light after a round trip through these devices will not be rotated.
In operation, the Faraday rotator of 18 and optically active element 20 act as a function group that rotates polarization 90 degrees for light traveling from left to right (from 16 to 22) but doesn""t rotate polarization for the light passing through from right to left (from 22 to 16). Similarly, optically active element 30 and faraday rotator 28 act as another function group with similar functionality. Input light has random polarization and includes two components. Polarizer prisms 14 or 22 reflect one component of the input light while another component passes through undeflected. For the purposes of this example, the first polarization P can be characterized as having a polarization that is in the incident plane (paper surface) and the second polarization S which polarization is perpendicular to the incident plane. The P components pass through polarizer prism 14 or 22, but S components reflect 90 degrees at an intersection to the surface. More specifically, a light with random SOP (State of Polarization) input to port 1 and transmitted to prism 14, divides into S and P components. The P components pass through to a second path (including components 12, 30, 28, 26 and 22), while the S components reflect to a first path (including components 14, 16, 18, 20 and 22).
For signal from port A to port B, the S components pass along the first path through the functional group of 18 and 20, change polarization to be P, passes through polarizer prism 22 to port B. The P component from port A, passes through polarizer prism 14, changes to be S polarization by functional group 30 and 28, and then reflects at polarizer prism 22 to port B also. Accordingly, polarizer prism 14 acts as a splitter while polarizer prism 22 acts as a combiner, producing the full signal from port A to port B.
For signal from port B to port C, the S components arriving at port B are reflected to the second path, pass through functional group of 28 and 30, maintain their S polarization, and are reflected at polarizer prism 14 to port C. The P component of the input light introduced at port B passes through polarizer prism 22 to the first path, passes through functional group of 18 and 20, maintains the P polarization, and then passes through polarizer prism 14 to port C also. In this case, polarizer prism 22 is a splitter and polarizer prism 14 is a combiner. Thus the full signal from port B is received by port C. Similarly, the full signal from port C is delivered to port D and the full signal from port D to port A.
Optical circulators of this type are very difficult to manufacture. The difficulty arises in the perfectly parallel optical paths that must be maintained in the device (i.e., paths between polarizer prisms 14 and 22). At the present time, no such devices are commercially offered.
In one aspect the invention provides a closed loop optical circulator including a first port, a last port and means for establishing a last optical path where the last optical path provides a path from the last port to the first port The means for establishing includes two pairs of complementary crystals. Each crystal of a respective pair transmits an optical signal of one polarization without deflection and deflects an optical signal of another polarization. The first pair of complementary crystals deflects optical signals of a second polarization in a direction perpendicular to a plane of a page and receives an optical signal from the last port and transmits the optical signal to the first port. The second pair of complementary crystals operable deflects optical signals of a first polarization in a direction along the plane of the page and is disposed between the first pair of complementary crystals. The optical circulator includes two pairs of complementary half wave plate rotators. Each pair of complementary half wave plate rotators is disposed between a crystal of the first pair and a crystal of the second pair of complementary crystals. Each half wave plate rotator includes a pair of half wave plate rotator groups where it each group includes a half wave plate and a glass portion. The optical circulator includes a half wave plate and a Faraday rotator disposed between crystals of the second pair of complementary crystals.
In another aspect, the invention provides a closed loop optical circulator including a first port, a last port and a path between the two including two pairs of complementary crystals. Each crystal of a respective pair transmits an optical signal of one polarization without deflection and deflects an optical signal of another polarization. The first pair of complementary crystals deflects optical signals of a second polarization in a direction perpendicular to a plane of a page and receives an optical signal from the last port and transmits the optical signal to the first port. The second pair of complementary crystals deflects optical signals of a first polarization in a direction along the plane of the page and disposed between the first pair of complementary crystals. The optical circulator includes two pairs of complementary half wave plate rotators. Each pair of complementary half wave plate rotators is disposed between a crystal of the first pair and a crystal of the second pair of complementary crystals. Each half wave plate rotator includes a pair of half wave plate rotator groups where each group includes a half wave plate and a glass portion. The optical circulator includes a half wave plate and a Faraday rotator disposed between crystals of the second pair of complementary crystals.
In another aspect, the invention provides a closed loop optical circulator including a plurality of ports and a like plurality of paths. Each path couples a pair of ports, where light incident at a port is transmitted along a path to a next port in the closed loop circulator. The paths include a first crystal for splitting an input light signal into two components, a second crystal for deflecting the two components received from the first crystal in a direction if the two components have a first polarization, a third crystal for deflecting the two components received from the second crystal in an opposite direction if the two components have the first polarization, and a fourth crystal for joining the two components received from the third crystal.
Aspects of the invention can include one or more of the following features. The crystals can be constructed from birefringent material. The second and third crystals can be Yvo4 crystals. The first pair of complementary half wave plate rotators can include a first half wave rotator group having a half wave plate covering a second and third quadrants and a glass plate covering a first and fourth quadrants and a second half wave rotator group having a half wave plate covering a third and fourth quadrants and a glass plate covering a first and second quadrants. The second pair of complementary half wave plate rotators can include a first half wave rotator group having a half wave plate covering a third and fourth quadrants and a glass plate covering a first and second quadrants and a second half wave rotator group having a half wave plate covering a first and fourth quadrants and a glass plate covering a second and third quadrants.
In another aspect, the invention provides a closed loop optical circulator including a first crystal for splitting an input light signal into two components, a second crystal for deflecting the two components received from the first crystal in a direction if the two components have a first polarization, a third crystal for deflecting the two components received from the second crystal in an opposite direction if the two components have the first polarization, and a fourth crystal for joining the two components received from the third crystal.
In another aspect, the invention provides a closed loop optical circulator including first, second, third and fourth ports. The optical circulator includes a first crystal splitting an input light signal received at the first and third ports into two components respectively, and joining input light components received from each of the second and fourth ports respectively into output light signals. The optical circulator includes a second crystal deflecting the two components received from the first crystal in a direction for signals from the first port, while not reflecting signals from the third port, and deflecting the two components received from a third crystal in an opposite direction for signals from the fourth port, while not reflecting signals from the second port. The third crystal deflects the two components received from the second crystal in an opposite direction for signals from the first port while not reflecting signals from the third port, and deflects the two components received from a fourth crystal in an opposite direction for signals from the second port, while not reflecting signals from the fourth port. The fourth crystal splits an input light signal received at the second and fourth ports into two components respectively, and joins input light components received from each of the first and second ports respectively into output light signals.
Aspects of the invention can include one or more of the following advantages. The present invention provides an easily manufacturable optical circulator with a loop function such that an optical signal input at a last port is returned to a first port in the device. Other advantages will be readily apparent from the attached figures and the description below.